Method and device for signal transmission in a multi-node system

ABSTRACT

Provided are a method and apparatus for transmitting a signal of a multi-node system employing a plurality of nodes distributed geographically and a base station that can be controlled by being connected with each of the plurality of nodes. The method includes: mapping a plurality of codewords to a plurality of streams; mapping the plurality of streams to at least one node among the plurality of nodes; performing multiple input multiple output (MIMO) precoding to map the streams mapped to the at least one node to at least one transmit antennas; and transmitting a signal subjected to the MIMO precoding to a user equipment.

TECHNICAL FIELD

The present invention relates to wireless communications, and moreparticularly, to a method and apparatus for transmitting a signal in amulti-node system.

BACKGROUND ART

A data transfer amount of a wireless network has been rapidly increasedin recent years. It is because various devices, e.g., a smart phone, atablet personal computer (PC), or the like, that requiremachine-to-machine (M2M) communication and a high data transfer amounthave been introduced and distributed. To satisfy the required high datatransfer amount, a carrier aggregation (CA) technique, a cognitive radiotechnique, or the like for effectively using more frequency bands and amultiple antenna technique, a multiple base station cooperationtechnique, or the like for increasing data capacity within a limitedfrequency have recently drawn attention.

In addition, the wireless network has been evolved in a direction ofincreasing density of nodes capable of accessing to an area around auser. Herein, the node implies an antenna (or antenna group) which isseparated from a distributed antenna system (DAS) by more than a certaindistance. However, the node is not limited to this definition, and thuscan also be used in a broader sense. That is, the node may be apico-cell eNB (PeNB), a home eNB (HeNB), a remote radio head (RRH), aremote radio unit (RRU), a relay, a distributed antenna (group), etc. Awireless communication system having nodes with higher density canprovide higher system performance by cooperation between the nodes. Thatis, better system performance can be achieved when one base stationcontroller manages transmission and reception of respective nodes andthus the nodes operate as if they are antennas or an antenna group forone cell, in comparison with a case where the respective nodes operateas an independent base station (BS), advanced BS (ABS), Node-B (NB),eNode-B (eNB), access point (AP), etc., and thus do not cooperate witheach other. Hereinafter, a wireless communication system including aplurality of nodes is referred to as a multi-node system.

If each node of the multi-node system performs scheduling and handoverby having its own identifier (ID), such a multi-node system can beregarded as a multi-cell system. If a coverage of each cell (i.e., node)is overlaid in the multi-cell system, such a multi-cell system is calleda multi-tier network.

The multi-node system can use various transmission and receptiontechniques such as a technique in which a BS transmits and receives databy selecting a plurality of nodes and a technique in which a userequipment transmits and receives data from a plurality of nodes. In thiscase, there is a need for a method capable of effectively transmitting asignal according to a technique of transmitting data by using aplurality of nodes having a high transfer rate among these nodes.

SUMMARY OF INVENTION Technical Problem

The present invention provides a signal transmission method andapparatus in a multi-node system.

Technical Solution

According to an aspect of the present invention, a method oftransmitting a signal of a multi-node system employing a plurality ofnodes and a base station that can be controlled by being connected witheach of the plurality of nodes is provided. The method includes: mappinga plurality of codewords to a plurality of streams; mapping theplurality of streams to at least one node among the plurality of nodes;performing multiple input multiple output (MIMO) precoding to map thestreams mapped to the at least one node to at least one transmitantennas; and transmitting a signal subjected to the MIMO precoding to auser equipment.

According to another aspect of the present invention, an apparatus fortransmitting a signal of a multi-node system employing a plurality ofnodes and a base station that can be controlled by being connected witheach of the plurality of nodes. The apparatus includes: acodeword-stream mapper for mapping a plurality of codewords to aplurality of streams; a stream-node mapper for mapping the plurality ofstreams to at least one node among the plurality of nodes; and a MIMOprecoder for performing MIMO precoding to map the streams mapped to theat least one node to a plurality of transmit antennas.

According to another aspect of the present invention, a method oftransmitting a signal of a multi-node system employing a plurality ofnodes and a base station that can be controlled by being connected witheach of the plurality of nodes is provided. The method includes: mappinga plurality of codewords to at least one node among the plurality ofnodes; mapping a codeword mapped to the at least one node to a pluralityof streams; performing MIMO precoding to map the streams mapped to theat least one node to a plurality of transmit antennas; and transmittinga signal subjected to the MIMO precoding to a user equipment.

Advantageous Effects

The conventional wireless communication system is based on a centralizedantenna system. In the centralized antenna system, a multiple inputmultiple output (MIMO) precoding matrix is defined for up to 8 transmitantennas, and a precoding matrix index (PMI), channel qualityinformation (CQI), etc., fed back by a user equipment are determined. Ina multi-node system such as a distributed antenna system, the number oftransmit antennas may be greater than 8, and an antenna configuration ateach node must be taken into consideration. According to the presentinvention, the user equipment can feed back the CQI, the PMI, etc.,without an excessive increase in a feedback overhead in the multi-nodesystem. In addition, efficiency of the multi-node system can beincreased.

DESCRIPTION OF DRAWINGS

FIG. 1 shows an example of a multi-node system.

FIG. 2 shows a radio access structure of the conventional wirelesscommunication system.

FIG. 3 shows a radio access structure of a wireless communication systemto which the concept of a base transceiver system (BTS) hotel isapplied.

FIG. 4 is a block diagram showing an exemplary structure of atransmitter included in a centralized antenna system.

FIG. 5 shows a signal transmission system according to an embodiment ofthe present invention.

FIG. 6 shows a method of performing communication in a multi-node systemusing a structure of a signal transmission system of FIG. 5.

FIG. 7 shows a signal transmission system according to anotherembodiment of the present invention.

FIG. 8 shows a signalling process between a base station and a userequipment when a multiple input multiple output (MIMO) precoding matrixincludes a per-node power factor.

FIG. 9 shows a signal transmission system according to anotherembodiment of the present invention.

MODE FOR INVENTION

The technology described below can be used in various multiple accessschemes such as code division multiple access (CDMA), frequency divisionmultiple access (FDMA), time division multiple access (TDMA), orthogonalfrequency division multiple access (OFDMA), single carrier frequencydivision multiple access (SC-FDMA), etc. The CDMA can be implementedwith a radio technology such as universal terrestrial radio access(UTRA) or CDMA2000. The TDMA can be implemented with a radio technologysuch as global system for mobile communications (GSM)/general packetratio service (GPRS)/enhanced data rates for GSM evolution (EDGE). TheOFDMA can be implemented with a radio technology such as institute ofelectrical and electronics engineers (IEEE) 802.11 (Wi-Fi), IEEE 802.16(WiMAX), IEEE 802.20, evolved UTRA (E-UTRA), etc. The UTRA is a part ofa universal mobile telecommunications system (UMTS). 3^(rd) generationpartnership project (3GPP) long term evolution (LTE) is a part of anevolved UMTS (E-UMTS) using the E-UTRA. The 3GPP LTE uses the OFDMA in adownlink and uses the SC-FDMA in an uplink. LTE-advance (LTE-A) isevolved from the LTE. The IEEE 802.11m is evolved from the IEEE 802.16e.

FIG. 1 shows an example of a multi-node system.

Referring to FIG. 1, the multi-node system includes a base station (BS)and a plurality of nodes.

The BS provides a communication service to a specific geographicalregion. The BS is generally a fixed station that communicates with auser equipment (UE) and may be referred to as another terminology, suchas an evolved Node-B (eNB), a base transceiver system (BTS), etc.

A distributed antenna is shown in FIG. 1 as an example of a node, and inthis sense, is denoted by an antenna node (AN). However, the node is notlimited to the distributed antenna, and thus may be, for example, amacro eNB antenna, a pico-cell eNB (PeNB), a home eNB (HeNB), a remoteradio head (RRH), a relay, etc. The node is also referred to as a point.

From the viewpoint of the UE, the node can be identified or indicated byusing a reference signal or a pilot signal. The reference signal (orpilot signal) is a signal known to a transmitting side and a receivingside, and implies a signal used for channel measurement, datademodulation, etc. For example, the reference signal may be a channelstatus indication reference signal (CSI-RS) defined in 3GPP LTE-A. In anLTE/LTE-A system, one CSI-RS configuration may be mapped to one node. Onthe basis of the CSI-RS configuration, the UE can identify or indicate anode and can obtain channel state information on the node. Byconsidering this, the node or the point can be replaced with the CSI-RSconfiguration in the present invention described below. The CSI-RSconfiguration may include information regarding the number of antennaports, a resource element (RE) in use, a transmission period, an offsetof a transmission time, etc.

The AN is connected to the BS in a wired/wireless fashion, and each ANmay consist of one antenna or an antenna group (i.e., a plurality ofantennas). Antennas belonging to one AN may be geographically locatedwithin several meters and show the same feature. In the multi-nodesystem, the AN serves as an access point (AP) to which the UE canaccess.

In a case where the node consists of antennas in the multi-node systemas described above, it may be called a distributed antenna system (DAS).That is, the DAS is a system in which antennas (i.e., nodes) aredeployed in various positions in a geographically distributed manner,and these antennas are managed by the BS. The DAS is different from aconventional centralized antenna system (CAS) in which antennas of theBS are centralized in a cell center.

If the antennas are deployed in a geographically distributed manner, itmay imply that, if one receiver receives the same signal from antennas,the antennas are deployed such that a channel state difference betweeneach antenna and the receiver is greater than or equal to a specificvalue. If the antennas are deployed in a centralized manner, it mayimply that the antennas are deployed in a localized manner such that achannel state difference between each antenna and one receiver is lessthan a specific value (the same is also true when a transmission entityis a BS). The specific value can be determined variously according to afrequency, service type, etc., used by the antennas.

FIG. 2 shows a radio access structure of the conventional wirelesscommunication system.

Referring to FIG. 2, the conventional wireless communication system maybe a cellular system. In the cellular system, a BS controls threesectors (e.g., 201, 202, and 203) constituting a cell, and each BS isconnected to a base station controller/radio network controller(BSC/RNC, hereinafter, collectively called a BSC) via a backbone network204. In the conventional wireless communication system, each BS isdeployed in a cell controlled by itself in general.

FIG. 3 shows a radio access structure of a wireless communication systemto which the concept of a BTS hotel is applied.

Referring to FIG. 3, each of BTSs can be connected through ANs deployedin a distributed manner in cells, an optical fiber, etc., and therespective BTSs are installed in a specific region in a localized mannerinstead of being deployed in cells managed by the BTSs. When a pluralityof BTSs which manage such distributed cells are deployed and managed bygrouping the BTSs in a specific region, it is called a BTS hotel. BTSswhich are grouped and installed by the concept of the BTS hotel have anadvantage in that costs for a land, a building, etc., in which the BTSis installed can be decreased, and costs ofmaintenance/management/repair can be decreased. In addition, the BTSsand the BSC/RNC can be installed in one place all together to increasebackhaul capacity. The concept of the BTS hotel can be applied to adistributed antenna system.

FIG. 4 is a block diagram showing an exemplary structure of atransmitter included in a centralized antenna system.

Referring to FIG. 4, a transmitter 1500 may include modulation mappers1510-1, . . . , 1510-K, a layer mapper 1520, a layer permutator 1530,transform precoders (DFT units) 1540-1, . . . , 1540-N, a MIMO precoder1550, resource element mappers 1560-1, . . . , 1560-N, and signalgenerators 1570-1, . . . , 1570-N.

The modulation mappers 1510-1, . . . , 1510-K receive a codeword and mapthe codeword to a modulation symbol that expresses a location on asignal constellation. Herein, the codeword implies coded data obtainedby performing encoding according to a predetermined coding scheme.Although not shown, the codeword may be input to the modulation mappers1510-1, . . . , 1510-K after being subjected to scrambling. A codeword qcan be expressed by Equation 1 below.

b ^((q))(k)=[b ^((q))(0)b ^((q))(1) . . . b ^((q))(N _(bit)^((q))−1)]  [Equation 1]

In Equation 1, q denotes a codeword index, and N^((q)) _(bit) denotesthe number of bits of the codeword q. k has a value in the range of 0 toN^((q)) _(bit)−1.

A modulation scheme is not limited to a specific modulation scheme, andmay be an m-phase shift keying (m-PSK) or an m-quadrature amplitudemodulation (m-QAM). Examples of the m-PSK include binary PSK (BPSK),quadrature PSK (QPSK), and 8-PSK. Examples of the m-QAM include 16-QAM,64-QAM, and 256-QAM. A modulation symbol modulated by the modulationmapper has a complex value. The codeword q mapped to the symbol on thesignal constellation can be expressed by a modulation symbol sequence asexpressed by Equation 2 below.

d ^((q))(i)=d ^((q))(0), . . . , d ^((q))(M ^((q)) _(symb)−1)  [Equation2]

In Equation 2, q denotes a codeword index, and M^((q)) _(symb) denotesthe number of symbols of the codeword q.

The layer mapper 1520 receives a modulation symbol sequence (i.e.,d^((q))(i)) from the modulation mappers 1510-1, . . . , 1510-K andperforms codeword-to-layer mapping. The layer mapper can also be calleda codeword-stream mapper. A stream is the same concept as a layer inLTE/LTE-A. A modulation symbol x(i) on which the codeword-to-layermapping is performed can be expressed by Equation 3 below.

x(i)=[x ⁽⁰⁾(i), . . . , x ^((v-1))(i)]^(T)  [Equation 3]

In Equation 3, ν denotes the number of layers, and i=0, 1, . . . ,M^(layer) _(symb)−1

M^(layer) _(symb) denotes the number of modulation symbols per layer.

If the number of codewords is 1 or 2, codeword-to-layer mapping forspatial multiplexing can be performed as defined in Table 1 below.

TABLE 1 Number Number of of code- Codeword-to-layer mapping layers wordsi = 0, 1, . . . , M_(symb) ^(layer) − 1 1 1 x⁽⁰⁾(i) = d⁽⁰⁾(i) M_(symb)^(layer) = M_(symb) ⁽⁰⁾ 2 2 x⁽⁰⁾(i) = d⁽⁰⁾(i) M_(symb) ^(layer) =M_(symb) ⁽⁰⁾ = x⁽¹⁾(i) = d⁽¹⁾(i) M_(symb) ⁽¹⁾ 2 1 x⁽⁰⁾(i) = d⁽⁰⁾(2i)M_(symb) ^(layer) = M_(symb) ⁽⁰⁾/2 x⁽¹⁾(i) = d⁽⁰⁾(2i + 1) 3 2 x⁽⁰⁾(i) =d⁽⁰⁾(i) M_(symb) ^(layer) = M_(symb) ⁽⁰⁾ = x⁽¹⁾(i) = d⁽¹⁾(2i) M_(symb)⁽¹⁾/2 x⁽²⁾(i) = d⁽¹⁾(2i + 1) 4 2 x⁽⁰⁾(i) = d⁽⁰⁾(2i) M_(symb) ^(layer) =M_(symb) ⁽⁰⁾/2 = x⁽¹⁾(i) = d⁽⁰⁾(2i + 1) M_(symb) ⁽¹⁾/2 x⁽²⁾(i) =d⁽¹⁾(2i) x⁽³⁾(i) = d⁽¹⁾(2i + 1)

The layer permutator 1530 can perform modulation symbol levelpermutation (or interleaving) on the modulation symbol x(i) on whichcodeword-to-layer mapping is performed. Permutation may be performed ina unit of bit, in a unit of modulation order, in a unit of modulationorder×a DFT size, and in a unit of modulation order×DFT size×(the numberof SC-FDMA symbols of a slot or a subframe). When the modulation symbollevel permutation is performed, a modulation symbol y(i) to be sent toeach antenna port p is output as x(i). A modulation symbol on which themodulation symbol level permutation is performed is denoted by y(i).

That is, if the modulation symbol, i.e., x(i)=[x⁽⁰⁾(i), . . . ,x^((v-1))(i)]^(T), i=0, 1, . . . , M^(layer) _(symb)−1, on whichcodeword-to-layer mapping is performed is given as an input vector ofthe layer permutator 1530, the output vector, i.e., y(i)=[y⁽⁰⁾(i), . . ., y^((p-1))(i)]^(T), i=0, 1, . . . , M^(layer) _(symb)−1, on which themodulation symbol level permutation is performed is generated.

The transform precoders 1540-1, . . . , 1540-N receive the modulationsymbol y(i) on which the modulation symbol level permutation isperformed, and perform a DFT operation on the received symbol. The DFToperation and the permutation may be performed in two ways, i.e., (1)the DFT operation is performed after performing permutation, and (2) thepermutation is performed after performing the DFT operation.

The MIMO precoder 1550 processes an input symbol by using a MIMO schemeaccording to the multiple Tx antennas. That is, the MIMO precoder 1550can perform layer-to-antenna mapping. The MIMO precoder 1550 distributesan antenna-specific symbol to the resource element mappers 1560-1, . . ., 1560-N for a path of a specific antenna.

The resource element mappers 1560-1, . . . , 1560-N allocate theantenna-specific symbol to a proper resource element, and performmultiplexing according to a user. The signal generators 1570-1, . . . ,1570-N perform an inverse fast Fourier transform (IFFT) operation or aninverse Fourier transform (IFT) operation and thereafter perform digitalto analog conversion (DAC). The signal generators 1570-1, . . . , 1570-Nmay include an IFFT unit and a CP insertion unit. An analog signaloutput from the signal generators 1570-1, . . . , 1570-N is transmittedthrough a physical antenna port.

As described above, in the conventional wireless communication system,the transmitter includes a layer mapper for mapping a codeword to alayer (stream) and a MIMO precoder. In general, the maximum number oftransmissible streams is the same as the number of ranks of a channelbetween a transmitter and a receiver. A codeword (or a MIMO layer inIEEE 802.16) to which the same modulation coding scheme (MCS) is appliedcan be mapped to a plurality of streams. For example, in LTE-A, up totwo codewords transmitted to one UE can be mapped to up to 4 streams. InIEEE 802.16m, one codeword transmitted to one UE can be mapped to up to8 streams (in case of IEEE 802.16m, a MIMO encoder performs mappingbetween a codeword and a stream).

After performing the mapping between the codeword and the stream, MIMOprecoding is performed to map the stream to an antenna (called anantenna port in LTE-A). The MIMO precoding primarily uses linearprecoding. Therefore, if the number of streams is denoted by N_(s) andthe number of Tx antenna (or antenna ports) is denoted by N_(t), thenthe MIMO precoding can be expressed by an N_(s)×N_(t) matrix

However, in order to directly apply the aforementioned MIMO precoding tothe multi-node system, the total number of Tx antennas of all nodes inthe multi-node system is N_(t). Then, the UE must select and feed back aprecoding matrix index (PMI) for the N_(t) Tx antennas. The PMI providesinformation on a precoding matrix suitable for a channel incodebook-based precoding. The PMI may be a simple matrix index in acodebook.

Meanwhile, the value N_(t) may be various according to the number ofnodes included in the multi-node system and the number of Tx antennas ofeach node, and a greater number of Tx antennas can be provided incomparison with the conventional 8 Tx antennas. That is, the multi-nodesystem may have a greater number of Tx antennas than the conventionalCAS or may have a various number of Tx antennas, which results in aproblem in that the number of codebooks to be defined or the number ofMIMO precoding matrices is increased.

In addition, a feedback of a CQI is a feedback of quality of aneffective channel corresponding to each codeword, and if the multi-nodesystem uses a plurality of codewords, each codeword can be transmittedto a UE through some Tx antennas among all Tx antennas. In this case,there is a problem in that the UE cannot know to which node the some Txantennas are included. There is a need for a communication method andapparatus capable of solving this problem.

FIG. 5 shows a signal transmission system according to an embodiment ofthe present invention.

Referring to FIG. 5, the signal transmission system includes acodeword-stream mapper 401, a stream-node mapper 402, and MIMO precoders403-1, . . . , 403-N. That is, the signal transmission system of FIG. 5differs from the transmitter of FIG. 3 in that the stream-node mapper402 is added between the codeword-stream mapper 401 and the MIMOprecoders 403-1, . . . , 403-N.

The codeword-stream mapper 401 maps a codeword to a stream (or layer).The stream-node mapper 402 maps the stream to each node. That is, thestream-node mapper 402 takes a role of distributing streams to aplurality of nodes. The MIMO precoders 403-1, . . . , 403-N perform MIMOprecoding at the respective nodes. The MIMO precoders 403-1, . . . ,403-N can be implemented at the respective nodes.

The stream-node mapper 402 is necessary because streams transmitted to aspecific UE or a UE group can be transmitted in a plurality ofdistributed nodes instead of being transmitted in one node. If theplurality of distributed nodes transmit the streams, a rank is increasedin a channel with respect to the UE, and a signal to noise ratio (SNR)is increased, which may result in the increase in a throughput.

The stream-node mapper 402 can be characterized as follows.

1. The total number of streams allocated to each node is greater than orequal to the number of input streams.

2. The number of streams allocated to one node is less than or equal tothe number of input streams.

3. All input streams are mapped to at least one node.

4. A steam output to one nods is a subset of all input streams.

Assume that input streams input to the stream-node mapper 402 aredenoted by s=[s₁, . . . , s_(N) _(s) ]^(T), and streams output to ani^(th) node are denoted by s_(i)=[s₁′, . . . , s_(N) _(s,i) ′]^(T).Then, the input streams and the streams output to the i^(th) node can beexpressed by Equation 4 below.

s _(i) =U _(i) s  [Equation 4]

In Equation 4 above, U_(i) denotes an N_(s,i)×N_(s) matrix, and each rowof U_(i) consists of any 1×N_(s) unit vector. The unit vector is avector of which only one element is 1 and the remaining elements are 0.In addition, U_(i) does not have any two rows identical to each other.Therefore, a rank of the matrix U_(i) is N_(s,i).

For example, assume that the stream-node mapper 402 maps three streamsto two nodes (i.e., a node 1 and a node 2). Then, U_(l) which maps astream to the node 1 and U₂ which maps a stream to the node 2 can beexpressed by Equation 5 below.

$\begin{matrix}{{U_{1} = \begin{bmatrix}1 & 0 & 0 \\0 & 1 & 0\end{bmatrix}},{U_{2} = \begin{bmatrix}0 & 0 & 1 \\0 & 1 & 0\end{bmatrix}}} & \lbrack {{Equation}\mspace{14mu} 5} \rbrack\end{matrix}$

U₁ and U₂ of Equation 5 above show an example in which, s₁ and s₂ aremapped to the node 1 and s₃ and s₂ are mapped to the node 2 in anorderly manner among three streams s_(l), s₂, and s₃. Then, MIMOprecoding suitable for an input consisting of s_(i) and s₂ is performedat the node 1, and MIMO precoding suitable for an input consisting of s₃and s₂ is performed at the node 2. In case of applying linear precoding,MIMO precoding can be performed by using an N_(s,i)ΔN_(t,i) matrix at ani^(th) node. Herein, N_(t,i) denotes the number of Tx antennas of thei^(th) node.

As described above, stream-node mapping information indicating mappingbetween a stream and a node can be expressed in a matrix. Thestream-node mapping information can be signaled by a BS to a UE or canbe fed back by the UE to the BS. For this, a mapping relation betweenthe stream and the node can be predetermined, and a signaling overheadcan be decreased by providing the stream-node mapping information in anindex format with respect to the predetermined mapping relation.

Table 2 below shows an example of indicating stream-node mappinginformation in an index format if the number of streams is 3 (i.e.,N_(s)=3) and the number of nodes is 2.

TABLE 2 Index U₁ U₂ 0 $\quad\begin{bmatrix}1 & 0 & 0 \\0 & 1 & 0\end{bmatrix}$ [0 0 1] 1 $\quad\begin{bmatrix}1 & 0 & 0 \\0 & 1 & 0\end{bmatrix}$ $\quad\begin{bmatrix}0 & 1 & 0 \\0 & 0 & 1\end{bmatrix}$ 2 $\quad\begin{bmatrix}1 & 0 & 0 \\0 & 1 & 0\end{bmatrix}$ $\quad\begin{bmatrix}1 & 0 & 0 \\0 & 0 & 1\end{bmatrix}$ 3 $\quad\begin{bmatrix}1 & 0 & 0 \\0 & 0 & 1\end{bmatrix}$ $\quad\begin{bmatrix}0 & 1 & 0 \\0 & 0 & 1\end{bmatrix}$ 4 $\quad\begin{bmatrix}1 & 0 & 0 \\0 & 1 & 0 \\0 & 0 & 1\end{bmatrix}$ [0 0 1] 5 $\quad\begin{bmatrix}1 & 0 & 0 \\0 & 1 & 0 \\0 & 0 & 1\end{bmatrix}$ $\quad\begin{bmatrix}0 & 1 & 0 \\0 & 0 & 1\end{bmatrix}$ 6 $\quad\begin{bmatrix}1 & 0 & 0 \\0 & 1 & 0 \\0 & 0 & 1\end{bmatrix}$ $\quad\begin{bmatrix}1 & 0 & 0 \\0 & 1 & 0 \\0 & 0 & 1\end{bmatrix}$ 7 $\quad\begin{bmatrix}1 & 0 & 0 \\0 & 1 & 0 \\0 & 0 & 1\end{bmatrix}$ None

In the example of Table 2 above, a node for transmitting more streams isfixed to the node 1 to decrease the number of indices. Therefore,additional signaling may be necessary to determine which node willtransmit more streams. In a first case, such signaling can be includedin information for reporting an order of a preferred node by a UE to aBS explicitly or implicitly. For example, if the UE feeds back CQI orpath loss information for each node, the preferred node can bedetermined implicitly by the CQI or path loss information.Alternatively, the UE may explicitly report the preferred node. In asecond case, such signaling can be included in control informationreported by the BS to the UE. The control information implies downlinkcontrol information (DCI), and can be transmitted through a controlchannel such as a physical downlink control channel (PDCCH), a physicaldownlink shared channel (PDSCH), A-MAP, etc.

In Table 2 above, an index 7 uses only a node 1, and such an index maybe unnecessary since it is enough to decrease the number of supportednodes. However, if the number of supported nodes is fixedsemi-statically, such an index may be necessary to off one nodeinstantaneously.

The stream-node mapping information may be given in a bitmap formatwithout being limited to the index format. If a system is configuredsuch that the same stream is not mapped to a plurality of nodes, abitmap format can be used to indicate a specific node to which aspecific stream is mapped, without having to use Table 2 above. Forexample, assume that three streams are mapped to two nodes. If each bitof a bitmap is 0, it may indicate mapping to a first node, and if eachbit is 1, it may indicate mapping to a second node. Then, if bitmapinformation of {011} is given to three streams, it may indicate that afirst stream is mapped to the first node, and second and third streamsare mapped to the second node.

Alternatively, stream-node mapping information may be configured in aformat of indicating an index of a stream mapped to each node. Forexample, the UE can feed back to the BS the stream-node mappinginformation indicating contents in which a stream 1 is mapped to a node1, and streams 1 and 2 are mapped to a node 2. Of course, the BS canalso transmit to the UE the aforementioned format of stream-node mappinginformation by including the information to control information.

In the multi-node system which uses the signal transmission system ofFIG. 5, the UE must feed back not a PMI for all nodes but a PMI for eachnode allocated to the UE when feeding back the PMI to the BS. By feedingback a per-node PMI, the BS can apply a MIMO precoder corresponding to aPMI for each node without having to configure a complex codebook andMIMO precoder considering a Tx antenna configuration of all nodes,thereby achieving a simple and clear system configuration.

FIG. 6 shows a method of performing communication in a multi-node systemusing a structure of the signal transmission system of FIG. 5.

Referring to FIG. 6, a BS transmits first stream-node mappinginformation to a UE (step S110). A node n transmits a stream set n1 tothe UE (step S121). A node m transmits a stream set m1 to the UE (stepS122). The stream set n1 and the stream set m1 are transmitted accordingto the first stream-node mapping information.

The UE transmits preferred node information to the BS (step S130). TheBS transmits second stream-node information by determining a stream-nodemapping relation to be applied to the UE on the basis of the preferrednode information (step S140). The node n1 and the node m1 transmit thestream set n2 and the stream set m2 respectively according to the secondstream-node mapping information (steps S141 and S142).

Although the multi-node system can perform a process of mapping acodeword to a node by using a codeword-stream mapper and a stream-nodemapper, the present invention is not limited thereto. In other words,the process of mapping the codeword to the node can be performed by thecodeword-stream mapper. That is, the codeword-stream mapper may mapstreams respectively to a plurality of nodes. Then, a MIMO precoder canperform MIMO precoding by using the streams mapped to the respectivenodes.

FIG. 7 shows a signal transmission system according to anotherembodiment of the present invention.

FIG. 7 differs from FIG. 5 in that a codeword-stream mapper 701 maps aninput codeword to a stream for each node. A MIMO precoder 702 performsMIMO precoding on the stream for each node.

Assume that N_(s) streams output by the codeword-stream mapper 701 aredenoted by s=[s₁ s₂ . . . s_(N) _(s) ]^(T), and Nt outputs which aresubjected to MIMO precoding by the MIMO precoder 702 are denoted byx=[x₁ x₂ . . . x_(N) _(t) ]^(T). If the MIMO precoder 702 uses linearprecoding, a MIMO precoding matrix V can be expressed by an N_(t)×N_(s),matrix, and has a relation of x=V s.

If the number of Tx antennas of a node i is denoted by N_(t,i), anN_(t,i)×1 vector x_(i) which is transmitted at each node can beconfigured by dividing elements of x respectively into N_(t,1), N_(t,2),. . . , N_(t,N) parts. Herein, i=1, 2, . . . , N. Therefore, a relationof x=[x₁ ^(T) x₂ ^(T) . . . x_(N) ^(T)]^(T) is satisfied. Likewise, aMIMO precoding matrix V_(i) corresponding to each node can be configuredby dividing each row of the MIMO precoding matrix V into N_(i,1),N_(t,2), . . . , N_(t,N) rows, respectively. That is V=[V₁ ^(T) V₂ ^(T). . . V_(N) ^(T)]^(T), Therefore, a Tx vector x_(i) transmitted at eachnode and a MIMO precoding matrix V_(i) at each node can be expressed byEquation 6 below.

x _(i) =V _(i) s,i=1, . . . , N  [Equation 6]

In a matrix V_(i) of Equation 6 above, all columns have zero vectorexcept for columns corresponding to streams allocated to an i^(th) node.For example, it is assumed that, in any system consisting of two nodes(i.e., N_(t,i)=N_(t,2)=4, N_(s)=4), streams 1 and 2 are mapped to afirst node and streams 2, 3, and 4 are mapped to a second node. Thirdand fourth columns of a 4×4 precoding matrix V₁ applied to a node 1 haveelements of 0, and a 1st column of a 4×4 precoding matrix V₂ applied toa node 2 have elements of 0.

Assume that {tilde over (V)}_(i) denotes a MIMO precoding matrix reducedto an N_(t,i)×N_(s,i) size by eliminating columns having a value 0 froma MIMO precoding matrix V_(i). Herein, N_(5,1) denotes the number ofstreams transmitted at the i^(th) node. In addition, assume that, in aTx stream matrix s, only streams transmitted at the node i are gatheredto express an N_(s,i)×1 vector s_(i) (in the example above,s₁=[s₁s₂]^(T), s₂=[s₂s₃s₄]^(T)).

In this case, x_(i) can be expressed by Equation 7 below.

x _(i) ={tilde over (V)} _(i) s _(i) ,i=1, . . . , N  [Equation 7]

If H_(i) denotes an N_(r)×N_(t,i) matrix corresponding to an i^(th) nodein an N_(r)×N_(t) channel matrix H for the UE, a receive (Rx) signal yof the UE can be expressed by Equation 8 below.

$\begin{matrix}\begin{matrix}{y = {{Hx} + z}} \\{= {{HVs} + z}} \\{= {{\sum\limits_{i = 1}^{N}\; {H_{i}x_{i}}} + z}} \\{= {{\sum\limits_{i = 1}^{N}\; {H_{i}V_{i}s}} + z}} \\{= {{\sum\limits_{i = 1}^{N}\; {H_{i}{\overset{\sim}{V}}_{i}s_{i}}} + z}}\end{matrix} & \lbrack {{Equation}\mspace{14mu} 8} \rbrack\end{matrix}$

In Equation 8 above, z denotes a vector indicating an Rx noise andinterference. It is assumed that a channel has a frequency flatcharacteristic in a specific narrowband.

According to Equation 8, the Rx signal y of the UE can be expressed by asum of products of a channel matrix H_(i) for the UE and the node i, aMIMO precoding matrix {tilde over (V)}_(i), and a Tx stream vectors_(i). That is, the Rx signal of the UE can be expressed in a form of asum of signals received from respective nodes, and is affected by theMIMO precoding matrix {tilde over (V)}_(i).

By considering this, the MIMO precoder in the multi-node system canconstitute a MIMO precoding matrix by including a per-node power factor.That is, a MIMO precoding matrix V_(i) of a node i can be expressed byEquation 9 below.

V _(i)=α_(i) {circumflex over (V)} _(i) ,i=1, . . . , N  [Equation 9]

In Equation 9, α_(i) denotes a power factor at a node i. Equation 9 hasthe same meaning as {tilde over (V)}_(i)=α_(i) V _(i), i=1, . . . , N.Herein, {circumflex over (V)}_(i) denotes an N_(t,i)×N_(s) matrix, and V_(i) denotes an N_(t,i)×N_(s,i) matrix. The matrix {circumflex over(V)}_(i) or V _(i) may be a MIMO precoding matrix defined based on theconventional CAS or a default MIMO precoding matrix in newly definedsingle node transmission.

The default MIMO precoding matrix in single node transmission uses atransmission method in which a power factor is fixed to 1. In this case,{circumflex over (V)}_(i) or V _(i) is characterized in that Tx power isnormalized. For example, if codebook-based precoding is applied,{circumflex over (V)}_(i) or V _(i) can be obtained from all normalizedcodebooks. In the normalized codebook, each element and each row orcolumn of a matrix and power of the matrix itself are fixed.

FIG. 8 shows a signalling process between a BS and a UE when a MIMOprecoding matrix includes a per-node power factor.

Referring to FIG. 8, the BS transmits a signal via a plurality of nodesby applying a first power factor (step S201). The first power factorcollectively indicates power factors for nodes allocated to the UE. Forexample, the BS can transmit a signal to the UE via a node 1 and a node2 by using the first power factor.

The UE measures a channel state for each of a plurality of nodes (stepS202), and feeds back a preferred power factor for each of the pluralityof nodes to the BS (step S203).

The UE can transmit the preferred power factor with the fixed number ofbits, and a predefined table can exist between the BS and the UEaccording to a bit value. Then, the BS can recognize a value based on abit value in the table. For example, the UE can transmit power factorinformation consisting of 2 bits for each node to the BS. If the bitvalue of 2 bits is ‘00’, ‘01’, ‘10’, or ‘11’, then the power factor canbe determined respectively to ‘0’, ‘0.5’, ‘1’, or ‘2’.

Alternatively, the UE can feed back information on the power factor inan event-driven manner. For example, the UE can include a power controlfield and a node index (or a corresponding reference signal index) inthe information to be fed back. The power control field may consist ofone bit. The BS can decrease Tx power of a node indicated by the indexif a field value of the power control field is 1, and can increase theTx power of the node indicated by the node index if the field value ofthe power control field is 0.

For example, the UE can feed back an increase of a power factor for anode 1 if a channel state with respect to the node 1 is not good.Alternatively, the UE can feed back a decrease of a power factor for anode 2 if a channel state with respect to the node 2 is good.

The BS transmits a signal via a plurality of nodes by applying a secondpower factor (step S204). The second power factor can be determinedbased on a preferred power factor transmitted by the UE. The first powerfactor and the second power factor can be transmitted by being includedin control information transmitted by the BS.

The aforementioned method eventually discriminates downlink Tx power foreach node. The reason of discriminating the Tx power for each node is toachieve the following two purposes.

1. To apply a codeword of the same MCS in a plurality of nodes.

Since a path loss between each node and a UE is different from eachother in a multi-node system, link quality from each node may show asignificant difference. As one method for dealing with this situation, adifferent MCS is applied to each node. However, this method requires aCQI feedback for each codeword and control information signalling for anMCS, which results in a problem in that a signalling overhead isincreased. In addition, if a single MCS is applied in a situation inwhich a path loss is different, there is a problem in that an MCS for anode having the worst channel state is applied to all nodes.

To solve this problem, link quality needs to be equalized for all nodes.For this, a method can be used in which Tx power is increased for nodeshaving a poor channel state and Tx power is decreased for nodes having agood channel state among nodes allocated to a UE or among nodes selectedby the UE.

2. To Maximize Link Efficiency

If a different codeword is mapped for each node and thus a different MCSlevel is provided, a BS can allocate power to maximize link capacity. Itis more preferable to stop transmission itself and increase Tx power toapply a higher MCS level to a node having a good link state rather thanperforming transmission with a low transfer rate by applying a lower MCSlevel to nodes having a poor link state among nodes allocated by the BSor among nodes selected by a UE. That is, as well known in a MIMOcapacity theory, a water filling power control method is used forcapacity maximization.

Hereinafter, a signalling process between a BS and a UE will bedescribed for a case where a multi-node system uses the signaltransmission system of FIG. 7.

If N_(r) denotes the number of Rx antennas of the UE, N_(t) denotes thetotal number of Tx antennas of the multi-node system, and H_(i) denotesan N_(r)×N_(t,i) matrix corresponding to an i^(th) node in anN_(r)×N_(t) channel matrix H for the UE, an Rx signal y of the UE can beexpressed by Equation 10 below.

$\begin{matrix}\begin{matrix}{y = {{Hx} + z}} \\{= {{HVs} + z}} \\{= {{\sum\limits_{i = 1}^{N}\; {H_{i}x_{i}}} + z}} \\{= {{\sum\limits_{i = 1}^{N}\; {H_{i}V_{i}s}} + z}} \\{= {{\sum\limits_{i = 1}^{N}\; {H_{i}{\overset{\sim}{V}}_{i}s_{i}}} + z}}\end{matrix} & \lbrack {{Equation}\mspace{14mu} 10} \rbrack\end{matrix}$

If the multi-node system performs a MIMO operation according to theconventional CAS-based communication standard, signal transmission andMIMO precoding matrix selection are performed based on a PMI feedback ofa UE with respect to a MIMO precoding matrix V.

However, the multi-node system may have Tx antennas in more variousnumbers than a case of using the CAS. For example, assume that thenumber of Tx antennas of a node 1 is 4, the number of Tx antennas of anode 2 is 2, the number of streams mapped to the node 1 is 2, the numberof streams mapped to the node 2 is 1, and the maximum rank of theoverall multi-node system is 2. In this case, since the total number Ntof Tx antennas is 6 and the maximum rank is 2, a 6×2 MIMO precodingmatrix V is defined. To perform a codebook-based close-loop feedback, a6×2 codebook must be newly designed, and 1^(st) and 2^(nd) columns of5^(th) and 6^(th) rows of a MIMO precoding matrix in a codebook whichsatisfies the above example must be 0. This is because rank-1transmission is performed at the node 2. It is difficult to define acodebook which satisfies such various restrictions.

In order to solve this problem, a MIMO precoding matrix can be definedfor each node. That is, the MIMO precoder per node is defined andapplied. For this, the BS can transmit not only all rank values but alsoa ‘per-node rank value’ by including them to control information. Inaddition, the number of Tx antennas for each node and stream-nodemapping information can be transmitted by including them to the controlinformation.

For example, the BS can report to the UE a value N_(s,i) which is thenumber of streams mapped to an i^(th) node, where the value N_(s,1) canbe a rank value of the i^(th) node. In addition, the BS can report a Txstream vector s_(i) of an i^(th) node from a Tx stream matrix s by usingstream-node mapping information. Then, the UE can find a PMI for acorresponding node from a codebook consisting of N_(t,i)×N_(s,i) MIMOprecoding matrices. In this case, the UE can find a PMI suitable foreach node allocated to the UE by using Equation 10. The BS can configurea MIMO precoding matrix (i.e., a MIMO precoder) from per-node PMIinformation which is fed back by the UE.

If the control information does not include the stream-node mappinginformation, the UE can find a per-node PMI set and feed back a part orentirety of it by assuming various stream-node mapping. In this case, astream-node mapping relation can be included in feedback information inthe UE.

If the control information does not include a per-node rank value in theBS, the UE feeds back a preferred per-node rank value by assumingvarious per-node rank values. In addition, the UE feeds back to the BS aper-node PMI set for a rank value for each preferred node.

For example, if the BS determines an overall rank to 4 in a system inwhich a node 1 has 4 Tx antennas and a node 2 has 4 Tx antennas, the UEfinds a per-node PMI set for each combination by assuming variouscombinations by which two streams can be allocated to two nodes.Examples of nodes mapped to four streams in an orderly manner mayinclude {1,1,1,1}, {1,1,1,2}, {1,1,2,2}, etc. Per-node PMI sets arefound for such various combinations, and a PMI set for each of some orall of the most preferred nodes is fed back.

In this case, the UE may allow feedback information to includestream-node mapping information, a per-node rank value, and an overallrank value. The overall rank value is included in the feedback inaddition to the per-cell rank value because a sum of per-node rankvalues becomes greater than the full rank value when some of streams aremapped to multiple nodes.

If preferred stream-node mapping information is configured in a form ofspecifying a stream mapped to a node, the per-node rank value and theoverall rank value can be omitted. For example, if preferred stream-nodemapping information is given in such a manner that a stream index mappedto a node 1 is {1, 2} and a stream index mapped to a node 2 is {2, 3,4}, a per-node rank value for the node 1, a per-node rank value for thenode 2, and an overall rank value are implicitly indicated as 2, 3, and4, respectively. Therefore, the per-node rank value and the overall rankvalue can be omitted.

A feedback of the per-node rank value can be regarded as an implicitindication of an index feedback for a node preferred by the UE. The BScan perform dynamic node switching by using the per-node rank value fedback by the UE. If the BS allocates a node set consisting of N nodes tothe UE, the number of nodes preferred by the UE may be instantaneouslyless than N. In this case, if the UE sets a per-node rank value for anon-preferred node to 0 and feeds back it, the BS can change a node setfor supporting the UE.

For example, assume that the BS allocates three nodes semi-statically tothe UE by using control information. The UE can feed back a per-noderank value for the three nodes.

If the per-node rank value for the three nodes is indicated as {1, 1,2}, {1, 2, 2}, etc., the UE can feed back, for example, {1,0,3}. Thisimplies information indicating that it is desirable to support a rank 1for a first node and a rank 3 for a third node for the UE by excluding asecond node among three allocated nodes. The BS can perform dynamic nodeswitching on the basis of the per-node rank value fed back by the UE.Likewise, the BS can allow the per-node rank value to be included incontrol information, and can report node information which changesdynamically to the UE by setting a per-node rank value for a specificnode to 0.

Hereinafter, a feedback of channel quality information (CQI) of a UE ina multi-node system will be described. In a narrow sense, the CQI isinformation for reporting to a BS an MCS level that can be received withperformance within a predetermined reception error rate. Alternatively,in a broad sense, the CQI is information for reporting to the BS acurrent channel state. The CQI can be used by being classified into anaverage CQI, a differential CQI, a wideband CAI, a subband CQI, etc. Ingeneral, the UE measures CQI values for respective codewords, and feedsback all or some of the values. However, multi-user (MU) MIMOtransmission has an exception in that a CQI value for a preferred streamis measured and fed back together with preferred stream information(IEEE 802.16m open-loop MU-MIMO).

In a multi-node system, not only an overall CQI value for an overall Txantenna but also per-node CQI information can be included in feedbackinformation when a UE feeds back CQI. The per-node CQI informationimplies CQI information for some Tx antennas or some streams. Whenfeeding back the per-node CQI information, the per-node CQI informationmust be included instantaneously in feedback information for all nodessupporting the UE. This is because it is difficult to determine an MCSlevel for another node supporting the UE when using only per-node CQIinformation for some nodes supporting the UE. In particular, since apath loss is different for each node in the multi-node system, such acharacteristic is more apparent. The per-node CQI information can beused when the BS reports an MCS level applied to each node by usingdownlink control information.

If the multi-node system supports dynamic node switching or if aper-node CQI feedback period is relatively slow, a node configuration(or stream-node mapping) for a case where per-node CQI information isconfigured may be different from a node configuration (or stream-nodemapping) for a case of actual data transmission.

For example, assume that the BS allocates nodes 1, 3, and 4 to a UE 1,and the UE 1 feeds back per-node CQI information for these three nodes.The BS can transmit data by using only some nodes among the three nodeswhen actual data transmission is achieved for the UE 1 when the UErequests a data. That is, if a node for which the UE feeds back per-nodeCQI information differs from a node for which the BS transmits data at alater time, CQI information is mismatched.

Alternatively, there may be a situation in which the UE obtains andfeeds back per-node CQI information for a case where different onestream is mapped to each node, but some nodes transmit multiple streamsin actual data transmission. The mismatch of CQI information also occursin such a situation.

In order to avoid the occurrence of CQI mismatch, the UE can add a CQIcompensation value to a feedback. The CQI compensation value implies adifference between CQI values for a case where a transmission mode(i.e., the number of nodes for transmitting signals, stream-nodemapping) assumed when the UE feeds back CQI is different from a nodewhich is set when the BS actually transmits data. For example, when theUE sends three CQI values for three nodes by assuming that three nodesare allocated, a difference value indicating how much the CQI valuechanges can be transmitted together as the CQI compensation value whenonly specific two nodes or one node participate in transmission amongthe three nodes.

Alternatively, per-node CQI information can be defined for a case whereonly a corresponding node participates in transmission. The CQIcompensation value can be defined as information indicating how much theper-node CQI information is changed due to an interference if anothernode also supports the UE together.

The CQI compensation value is obtained by assuming various cases, andthus can be fed back only with a relatively long period, i.e.,semi-statically, or can be fed back only for some limited cases. Forexample, the CQI compensation value can be fed back by limiting to a CQIincrease amount for a case where the number of nodes is decreased by onenode in comparison with a node configuration when performing a currentCQI feedback.

In the multi-node system using the aforementioned signal transmissionsystems, the UE can estimate a channel matrix H_(i) (i=1, 2, . . . , N)by using a reference signal corresponding to each node. The UE uses thechannel matrix H_(i) to find and feed back an overall rank value, aper-node rank value, and a corresponding preferred per-node PMI.

In addition, in case of applying the per-node PMI, a per-node CQI valueis fed back by measuring a per-node MCS level applicable to each node.The CQI compensation value can be fed back together with the per-nodeCQI value.

In addition, the overall rank value and the per-node rank value can beincluded in control information transmitted from the BS to the UE. Inthis case, the UE fixes a parameter to the overall rank value and theper-node rank value transmitted from the BS, and thereafter finds andfeeds back the remaining parameter values to be fed back.

When the aforementioned signal transmission systems are implemented in amulti-node system, various stream-node mapping can exist, which mayresult in an excessive increase in a feedback overhead of the UE. If thefeedback overhead exceeds a range supported by the UE or the multi-nodesystem, the following method can be used to decrease the feedbackoverhead. 1. The BS can designate stream-node mapping information bytransmitting control information including the stream-node mappinginformation. 2. A default stream-node mapping relation is defined byusing a predetermined standard, and the UE feeds back a PMI by assumingthe default stream-node mapping if there is no special request of theBS.

Herein, the default stream-node mapping may be a mapping scheme in whichone stream is allocated to all nodes. That is, a default value of anoverall rank is equal to the total number of nodes, and a per-node rankvalue for each node is set to 1.

If there is no special request from the BS, the UE configures feedbackinformation by assuming that 1^(st), 2^(nd), . . . , N^(th) streams aremapped respectively to nodes 1, 2, . . . , N.

The per-node rank value is set to 1 because there is a high possibilitythat each node has significantly lower Tx power than the conventionalmacro cell tower in the multi-node system and thus there is a highpossibility that the number of Tx antennas in each node is not great. Inother words, a possibility that the per-node rank value is greater thanor equal to 2 is not great. In addition, there is a high possibilitythat a characteristic of a channel from each node installed at aphysically different location has a very low spatial correlation of achannel between the respective nodes, and thus there is a highpossibility that no problem occurs in transmission of independentstreams.

The UE can estimate a channel from each node and thereafter can find aper-node PMI for each node from a rank-1 codebook, that is, a codebookincluding N_(t,i)×1 vectors, and then can feed back the PMI. In thiscase, stream-node mapping information can be omitted from the feedbackinformation, and PMI and CQI feedback information can also be limited toa per-node PMI and per-node CQI when the number of nodes is equal to thenumber of allocated nodes, and thus a feedback overhead is not great.

FIG. 9 shows a signal transmission system according to anotherembodiment of the present invention.

Referring to FIG. 9, the signal transmission system includes acodeword-node mapper 601, codeword-stream mappers 602-1, . . . , 602-N,and MIMO precoders 603-1, . . . , 603-N.

The codeword-node mapper 601 maps all codewords to each node. That is,instead of first mapping a codeword to a stream and then mapping it toeach node, a codeword is mapped to a node. When the signal transmissionsystem is configured as shown in FIG. 9, codeword-node mappinginformation may be included in information fed back by a UE to a BS orcontrol information transmitted by the BS to the UE.

Mapping between a codeword and a node can be set as a requirement bydefault to allocate one codeword per node. In this case, signaling ofcodeword-node mapping information may be necessary only when thisrequirement is not met. Accordingly, a signaling overhead is decreased.

The codeword-node mapping information may include two fields, forexample, a message field A and a message field B. In this case, if themessage field A is 0, a codeword mapped to a node specified in themessage field B can be increased (in case of LTE-A, increased to 2), andif the message field A is 1, a codeword for a node set specified in themessage field B can be shared. The increasing in the codeword for thespecific node can be determined at the request of the UE or by thedecision of the BS if quality difference becomes significant betweenstreams mapped to the node and thus it is not suitable to use the sameMCS. The sharing of the codeword for the specific node set can berequested by using a control message by the BS or by using a feedbackmessage of the UE when node sets have similar quality and thus it isintended to decrease a signaling overhead by distributing one codewordto a plurality of nodes.

In the conventional standard, up to two codewords are allocated to theUE. However, each node may have a different path loss in a multi-nodesystem. Therefore, performance can be maximized when a different MCS isapplied to each node. For this, the number of supportable codewords ispreferably equal to the maximum number of nodes that can be supported toone UE in the multi-node system. In this case, a codeword-node mappercan be utilized instead of the stream-node mapper.

The aforementioned method and apparatus can be implemented withhardware, software, or combination thereof. In hardware implementation,the present invention can be implemented with one of an applicationspecific integrated circuit (ASIC), a digital signal processor (DSP), aprogrammable logic device (PLD), a field programmable gate array (FPGA),a processor, a controller, a microprocessor, other electronic units, andcombination thereof, which are designed to perform the aforementionedfunctions. In software implementation, the present invention can beimplemented with a module for performing the aforementioned functions.Software is storable in a memory unit and executed by the processor.Various means widely known to those skilled in the art can be used asthe memory unit or the processor.

While the present invention has been particularly shown and describedwith reference to exemplary embodiments thereof, it will be understoodby those skilled in the art that various changes in form and details maybe made therein without departing from the spirit and scope of theinvention as defined by the appended claims. The exemplary embodimentsshould be considered in descriptive sense only and not for purposes oflimitation. Therefore, the scope of the invention is defined not by thedetailed description of the invention but by the appended claims, andall differences within the scope will be construed as being included inthe present invention.

1. A method of transmitting a signal of a multi-node system employing aplurality of nodes and a base station that can be controlled by beingconnected with each of the plurality of nodes, the method comprising:mapping a plurality of codewords to a plurality of streams; mapping theplurality of streams to at least one node among the plurality of nodes;performing multiple input multiple output (MIMO) precoding to map thestreams mapped to the at least one node to at least one transmitantennas; and transmitting a signal subjected to the MIMO precoding to auser equipment.
 2. The method of claim 1, further comprising:transmitting first stream-node mapping information by the base stationto the user equipment, wherein the first stream-node mapping informationis information indicating a mapping relation between the plurality ofstreams and the at least one node.
 3. The method of claim 2, furthercomprising: receiving a feedback of preferred node information from theuser equipment, wherein the preferred node information includes mappinginformation between a stream and a node recommended by the userequipment.
 4. The method of claim 3, further comprising: transmittingsecond stream-node mapping information by the base station to the userequipment, wherein the second stream-node mapping information isdetermined based on the preferred node information.
 5. The method ofclaim 4, wherein the first stream-node mapping information and thesecond stream-node mapping information include an index of a codebookincluding predetermined mapping relations.
 6. The method of claim 4,wherein the preferred node information includes an index of a codebookincluding predetermined mapping relations.
 7. The method of claim 4,wherein the first stream-node mapping information and the secondstream-node mapping information are transmitted through a controlchannel.
 8. An apparatus for transmitting a signal of a multi-nodesystem employing a plurality of nodes and a base station that can becontrolled by being connected with each of the plurality of nodes, theapparatus comprising: a codeword-stream mapper for mapping a pluralityof codewords to a plurality of streams; a stream-node mapper for mappingthe plurality of streams to at least one node among the plurality ofnodes; and a MIMO precoder for performing MIMO precoding to map thestreams mapped to the at least one node to a plurality of transmitantennas.
 9. The apparatus of claim 8, wherein the stream-node mappertransmits first stream-node mapping information to the user equipment,and the first stream-node mapping information is information indicatinga mapping relation between the plurality of streams and the at least onenode.
 10. The apparatus of claim 9, wherein the stream-node mappertransmits second stream-node mapping information to the user equipment,and the second stream-node mapping information is determined based onpreferred node information fed back by the user equipment, and whereinthe preferred node information includes an index of a matrix selected bythe user equipment from a codebook including predetermined mappingmatrices.
 11. The apparatus of claim 10, wherein the first stream-nodemapping information and the second stream-node mapping informationinclude an index of a codebook including predetermined mappingrelations.
 12. The apparatus of claim 11, wherein the first stream-nodemapping information and the second stream-node mapping information aretransmitted through a control channel.
 13. A method of transmitting asignal of a multi-node system employing a plurality of nodes and a basestation that can be controlled by being connected with each of theplurality of nodes, the method comprising: mapping a plurality ofcodewords to at least one node among the plurality of nodes; mapping acodeword mapped to the at least one node to a plurality of streams;performing MIMO precoding to map the streams mapped to the at least onenode to a plurality of transmit antennas; and transmitting a signalsubjected to the MIMO precoding to a user equipment.
 14. A method oftransmitting a signal of a multi-node system for configuring andoperating a plurality of reference signals, the method comprising:mapping one or more codewords to antenna ports corresponding to at leastone reference signal among the plurality of reference signals; mappingthe mapped codewords to a plurality of streams; performing MIMOprecoding to the mapped streams; and transmitting a signal subjected tothe MIMO precoding to a user equipment.
 15. The method of claim 14,wherein the plurality of reference signals are identified by informationon the number of antenna ports, a resource element (RE) in use, atransmission period, and an offset of a transmission time.
 16. Themethod of claim 14, further comprising: feeding back stream-referencesignal mapping information by the user equipment to a base station. 17.The method of claim 16, further comprising: feeding back at least one ofa rank value per reference signal and rank values for two or morereference signals by the user equipment to the base station.
 18. Themethod of claim 14, further comprising: designating a rank for each ofthe plurality of signals and a rank for the total number of antennaports included in the plurality of reference signals by a base stationto the user equipment.